54 FERGUSON'S LECTURES. 



LECT. bodies, how unequal soever in their weights, will 



v^x-v^ balance one another in all situations : for, as the whole 



ascent of one is performed in the same time with the 



whole descent of the other, their respective velocities 



must be directly as the spaces they move through ; and 



the excess of weight in one body is compensated by the 



How to excess of velocity in the other. Upon this principle it 



compute . / i 



the power 1S eas y w compute the power of any mechanical engine, 

 ^ f e a jjy . whether simple or compound ; for it is but only finding 

 cal en- how much swifter the power movea than the weight 

 does (i.e. how much farther in the same time), and just 

 so much is the power increased by the help of the 

 engine. 



In the theory of this science, we suppose all planes 

 perfectly even, all bodies perfectly smooth, levers to 

 have no weight, cords to be extremely pliable, machines 

 to have no friction ; and in short, all imperfections must 

 be set aside until the theory be established ; and then, 

 proper allowances are to be made. 



< hanical "^ ie s i m P^ e machines, usually called mechanical powers, 

 powers, are six in number, viz. the lever, the wheel and axle, the 

 pulley, the inclined plane, the wedge, and the screws* 

 They are called mechanical powers, because they help 

 us mechanically to raise weights, move heavy bodies, 

 and overcome resistances, which we could not effect 

 without them. 



The lever. 1. A lever is a bar of iron or wood, one part of which 

 being supported by a prop, all the other parts turn up- 

 on that prop as their center of motion : and the velocity 

 of every part or point is directly as its distance from 

 the prop. Therefore, when the weight to be raised at 

 one end is applied to the power at the other to raise it, 



Note 31. If is generally admitted that the six mechanical powers 

 enumerated by our author may fairly be reduced to two, as the pulley 

 and the wheel may be referred to the lever; whilst the wedge and the 

 screw are but modifications of the inclined plane. 



